1. Field of the Invention
This invention is in the field of selective etching of silicon nitrides overlying a substrate of SiO.sub.2 and involves the use of an etchant gas including a fluorohydrocarbon and a substantial amount of CO.sub.2.
2. Description of the Prior Art
In conventional dry etching methods for etching silicon nitride films (usually designated Si.sub.3 N.sub.4 films) formed on an SiO.sub.2 film in the process of fabricating semiconductor devices, the etchant gas is usually a mixture of tetrafluoro methane (CF.sub.4) and 0.sub.2, sometimes containing an additional diluent gas such as argon. The apparatus for industrial practice of this dry etching method includes tunnel-type plasma etchers, parallel plate plasma etchers of the anode coupling type and chemical dry etching (CDE) of the separate discharge chamber type. When operating such dry etching apparatus using a conventional etchant, it is possible to etch the Si.sub.3 N.sub.4 film at a selective ratio (about 5:1) between the Si.sub.3 N.sub.4 and the underlying Si0.sub.2 film. However, in each case the etching is accomplished in the manner of isotropic etching because the reaction mechanisms inherent in the etching apparatus and particularly the mechanisms of reactions which occur when radicals formed by dissociation of CF.sub.4 attack the Si.sub.3 N.sub.4 and SiO.sub.2 films. As a result, side etching occurs beneath the mask.
With the recent trend toward further miniaturization of semiconductor devices, it has become difficult to satisfy fully the requirements of the Si.sub.3 N.sub.4 films by conventional isotropic etching. It will soon become indispensable to accomplish anisotropic etching in exact conformity with the pattern dimensions of the mask by employing reactive ion etching (RIE). When etching a Si.sub.3 N.sub.4 film laid on a SiO.sub.2 film by a conventional RIE technique, there is no alternative to using an etchant gas which also etches Si0.sub.2. Therefore, it is difficult to accomplish good selectivity of the etcl hing rate between the Si.sub.3 N.sub.4 film and the underlying SiO.sub.2 film. Trifluoro methane (CHF.sub.3) is well known as an etchant gas for reactive ion etching of SiO.sub.2. When, for example, O.sub.2 is added to CHF.sub.3 at a CHF.sub.3 /O.sub.2 ratio of 40/7 on a flow rate (sccm) basis to etch either Si.sub.3 N.sub.4 or SiO.sub.2 at a pressure of 0.06 Torr and a radio-frequency power of 400 W (0.20 W/cm.sup.2), the etching rate is 980 .ANG. for Si.sub.3 N.sub.4 and 510 .ANG. for SiO.sub.2. Thus, the Si.sub.3 N.sub.4 /SiO.sub.2 selective ratio is only 1.9 though anisotropic etching is accomplished without significant undercutting. Since the controllability must also be taken into consideration, this method is deemed to be impractical for a process in which good selectivity of etching between a silicon nitride film and an underlying silicon dioxide layer is required. For example, the same method is not useful for etching Si.sub.3 N.sub.4 which exists as a selective oxidation mask on a thin pad of SiO.sub.2.
Recently, an etching gas consisting of difluoro methane (CH.sub.2 F.sub.2) was reported in the International Electric Device Meeting, 1983, under the title "VLSI Device Fabrication Using a Unique, Highly-Selective Si.sub.3 N.sub.4 Dry Etching". This etchant attracted interest as a promising solution to the above described problem. However, some problems still remain unsolved as to the practical use of this etchant gas. For example, when this etchant gas is used under conditions of high selectivity, it often results in the production of a polymer film which is difficult to remove from the surface after etching. A considerable etching residue thus may adhere to the substrate surface after etching. It is probable that such phenomena are attributable to the fact that CH.sub.2 F.sub.2 has a lower C/F ratio than CF.sub.4 and CHF.sub.3 which are conventionally used for etching Si.sub.3 N.sub.4, and that the etchant gas contains H.sub.2 within the molecule. Because of the nature of CH.sub.2 F.sub.2, a considerably carbon-rich condition is produced in the plasma of etchant gas so that polymers of fluorocarbons are likely to be deposited on the surface after etching. A probable cause of the existence of etching residue on the substrate surface is an accumulation of such polymers. In addition, accumulation of the polymers in the chamber of the etching device is considerable. Due to these considerations, it is practically impossible to perform stable etching operations with good reproducibility using this type of etchant.
With respect to the commonly used fluorine-containing etchant gases such as CF.sub.4 and CHF.sub.3, it has been suggested to add a small amount of O.sub.2 CO.sub.2, viz. about 5% in most cases and up to about 10% at the maximum, for the purpose of suppressing formation of fluorocarbon polymers. This technique is based on the thought that oxygen radicals formed in the plasma of the mixed gas were removed carbon by converting it into CO and/or CO.sub.2 with the effect of increasing the F/C ratio in the plasma and thereby preventing deposition of the polymers. Additionally, the etch rate of Si.sub.3 N.sub.4 becomes higher when the etching gas contains a small amount of O.sub.2 or CO.sub.2. However, when using such a mixed gas for etching of Si.sub.3 N.sub.4 on top of SiO.sub.2, the etch rate of the underlying SiO.sub.2 also increases because of the suppression of the formation of the polymers which are effective to prevent etching of SiO.sub.2. Consequently, the selective ratio of etching between Si.sub.3 N.sub.4 and SiO.sub.2 becomes very much lower than the desired or tolerable level. This deficiency is not fundamentally removed even when CO.sub.2 is added to the etching gas instead of O.sub.2